A compact Selective Catalytic Reduction (SCR) system comprising a gas flow inlet system, a vaporizer module and an SCR reactor is described. The inlet flow system is configured to provide heat to the vaporizer module, to mix reductant with the exhaust gas and to provide an approximately uniform flow of the exhaust gas through the catalyst. The vaporizer module is configured to vaporize reductant from a solution of a reductant or a precursor of a reductant and to transfer the vaporized reductant into the gas flow inlet system, where it is mixed with exhaust gas. The SCR reactor contains and SCR catalyst is in fluid communication with the inlet flow system and the vaporizer module.